Silvaco uses cookies to improve your user experience and to provide you with content we believe will be of interest to you. Learn detailed information on Privacy Policy. By using this website, you consent to the use of our cookies.

Contact Us

TECHNICAL LIBRARY

Q: How can the reverse short channel effect
(RSCE) in MOSFETs be simulated using ATHENA and ATLAS? How can the
physical effect behind RSCE be tuned ?

A: RSCE in MOSFETs is where the threshold
voltage increases with decreasing channel length. At very short
channel lengths the normal short channel effect takes over and the
threshold voltage decreases.

The cause of the increasing threshold voltage
is a non-uniform enhancement of diffusion of the channel implant
laterally along the MOS channel. This non-uniformity arises from
the extra point defects generated in the source and drain areas
of the MOSFET. The source of these point defects is most commonly
the damage caused by the heavy n+ and LDD implants. Other possible
causes that can be modeled in ATHENA are oxidation or silicidation
of the source and drain area.

The amount of implant damage from the source/drain
implants is controlled using theDAM.FACTORparameter. The effect of the damage on subsequent
diffusions are modeled in ATHENA using the fully coupled diffusion
model (METHOD FULL.CPL).
A previous hints and tips covered a description of this (Simulation
Standard, Feb 1995).

To model RSCE in ATHENA and ATLAS it is
necessary to construct MOSFETs of different channel lengths. This
can be done either using the MaskViews layout interface, or using
theSTRETCHcommand in
ATHENA or DevEdit. The user should simulate the shortest channel
length up until the polysilicon etch and stretch the device to the
desired length. TheFULL.CPLmodel
is only required for diffusion after the source/drain implants.

Figure 1 shows the result of a threshold voltage simulation
versus gate length for various values of implant damage. VWF was used to automatically
generate and run this experiment. VWF handles the automatic interface to ATLAS
and the extraction of the threshold voltages. Looking horizontally along the
y=0 line it is seen that with zero implant damage the threshold voltage decreases
with decreasing length. No RSCE is seen. However asDAM.FACTis increased, the threshold voltage starts to rise before
falling at very short lengths. It is clear the size of the RSCE increases with
implant damage factor.

It is also interesting to note that even the threshold
voltage for the 20mm long device is affected slightly by the implant damage.
This is to be expected from Figure 2 on page 5 which shows point defects diffusing
30mm into the substrate. The lateral diffusion length of point defects should
be of a similar order.

Many parameters can be used to tune the fully coupled
diffusion model. The most effective for RSCE is the surface recombination of
the interstitials (KSURF.0). Figure
2 shows threshold voltage versus channel length as a function ofKSURF.0for a fixedDAM.FACT. High
values ofKSURF.0show no RSCE effect
while lower values show strong increases in threshold at lengths around 1.0
mm.

Tuning RSCE usingDAM.FACTandKSURF.0is possible using ATHENA, ATLAS and
VWF. Users should note that both these parameters will affect process simulation
results such as source/drain junction depth. Figure 3 shows a graph of junction
depth of an arsenic implant after a fixed diffusion as a function ofDAM.FACTandKSURF.0.For a given measured result for junction depth it is clear
there are a whole set ofDAM.FACTandKSURF.0combinations that can produce the
correct answer. However the effect of each combination that matches a junction
depth is not the same on RSCE.